Heatsink sizing calculator

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Device & thermal budget

Dissipated power (P)W — (Vin−Vout)·Iout for a regulator, Rds·I² for a MOSFET, etc.Max junction temperature (Tj)°C — 125 typical, 150 for some partsAmbient temperature (Ta)°C — internal enclosure air, worst casePackageJunction-to-case resistance, °C/WCase-to-sink resistance (Rθ_cs)°C/W — 0.5 with thermal grease, ~0.3 with a pad

Result

Total allowed thermal resistance

(Tj − Ta) / P

7.5 °C/W

Required heatsink resistance (Rθ_sa)

Rθ_total − Rθ_jc − Rθ_cs

4 °C/W

Design target (80% of requirement)

Pick a heatsink rated at or below this

3.2 °C/W

About the thermal chain

Heat flows junction → case → sink → ambient, and each link has a thermal resistance in °C/W. Summing them and equating to (Tj − Ta)/P gives the heatsink resistance you must not exceed. Heatsink datasheets rate Rθ_sa in free air (natural convection) or with a fan (forced airflow — usually a 3–5× improvement). The 80% design target leaves margin for mounting, dust and part-to-part variation; forced-air and 2-layer-board packages derate differently, so treat the number as a starting point for selection.

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